THE STRUCTURE OF THE ELEMENTARY TISSUES. 5? 



From these marrow-cells, especially those of the red marrow, are de- 

 rived, as we shall presently show, large quantities of red blood-corpuscles. 



Periosteum and Nutrient Blood-vessels. The surfaces of bones, 

 except the part covered with articular cartilage, are clothed by a tough, 

 fibrous membrane, the periosteum; and it is from the blood-vessels 

 which are distributed in this membrane, that the bones, especially their 

 more compact tissue, are in great part supplied with nourishment, 

 minute branches from the periosteal vessels entering the little foramina 

 on the surface of the bone, and finding their way to the Haversian 

 canals to be immediately described. The long bones are supplied also 

 by a proper nutrient artery which, entering at some part of the shaft so 



Fig. 60. Transverse section of compact bony tissue (of humerus). Three of the Haversian 

 canals are seen, with their concentric rings; also the lacunas, with the canaliculi extending from 

 them across the direction of the lamellae. The Haversian apertures were filled with debris in grind- 

 ing down the section, and therefore appear black in the figure, which represents the object. as viewed 

 with transmitted light. The Haversian systems are so closely packed in this section, that scarcely 

 any interstitial lamellae are visible. X 150. (Sharpey.) 



as to reach the medullary canal, breaks up into branches for the supply 

 of the marrow, from which again small vessels are distributed to the 

 interior of the bone. Other small blood-vessels pierce the articular 

 extremities for the supply of the cancellous tissue. 



Microscopic Structure of Bone. Notwithstanding the differences of 

 arrangement just mentioned, the structure of all bone is found under 

 the microscope to be essentially the same. 



Examined with a rather high power its substance is found to contain 

 a multitude of small irregular spaces, approximately fusiform in shape, 

 called lacunw, with very minute canals or canaliculi, as they are termed, 

 leading from them, and anastomosing with similar little prolongations 

 from other lacunae (fig. 60). In very thin layers of bone, no other 

 canals than these may be visible; but on making a transverse section of 



